1. Field of the Invention
The present invention relates to amplitude reduction and damping of vibration in structures, and, more particularly, to a passive piezoelectric vibration suppression and damping device.
2. Description of the Related Art
Piezoelectricity is the ability of some materials (notably crystals and certain ceramics, including bone) to generate an electric potential in response to applied mechanical stress. This may take the form of a separation of electric charge across the crystal lattice. If the material is not short-circuited, the applied charge induces a voltage across the material. In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal, overall, is electrically neutral. Each of these sides forms an electric dipole, and dipoles near each other tend to be aligned in regions called Weiss domains. The domains are usually randomly oriented, but can be aligned during “poling”, a process by which a strong electric field is applied across the material, usually at elevated temperatures.
When a mechanical stress is applied, this symmetry is disturbed, and the charge asymmetry generates a voltage across the material. For example, a 1 cm3 cube of quartz with 2 kN of correctly applied force can produce a voltage of 12,500 V. Piezoelectric materials also show the opposite effect, called the converse piezoelectric effect, where the application of an electrical field creates mechanical deformation in the crystal.
Piezoelectricity is the combined effect of the electrical behavior of the material, given by D=εE, where D is the electric charge density displacement (also referred to as the electric displacement), ε is permittivity and E is electric field strength, and Hooke's Law, which is given as S=sT, where S is strain, s is compliance and T is stress.
These may be combined into so-called “coupled equations”, of which the strain-charge form is given by {S}=└SE┘{T}+└dt┘{E} and {D}=└d┘{T}+└εt┘{E}, where [d] is the matrix for the direct piezoelectric effect and [dt] is the matrix for the converse piezoelectric effect. The superscript E indicates a zero, or constant, electric field; the superscript T indicates a zero, or constant, stress field; and the superscript t stands for transposition of a matrix.
Many materials, both natural and man-made, exhibit piezoelectricity, such as berlinite (AlPO4), a rare phosphate mineral that is structurally identical to quartz, cane sugar, quartz, Rochelle salt, topaz, and tourmaline-group minerals.
Structures, such as those used in aircraft and spacecraft, experience vibration during service. The vibration is undesirable because it is uncomfortable for occupants or can lead to control problems or damage to sensitive instruments in the vehicle. The vibration is also undesirable because it leads to fatigue damage of the structure itself. Care is taken in the design of the structure to minimize such vibration, but some vibration is always present regardless of the care taken in design.
A number of techniques have been developed to reduce the amplitude and achieve damping of the structural vibration. In one such common technique, energy-absorbing materials, such as elastomers, are built into the structure or into joints of the structure. These energy-absorbing materials reduce the amplitude of the vibration without damage to the materials.
In another approach, piezoelectric materials are used to convert mechanical vibrational energy into electrical energy, and the electrical energy is thereafter dissipated. A piezoelectric material is one that converts electrical energy into mechanical movement, or, conversely, converts mechanical movement such as vibration into an electrical voltage. A “patch” of the piezoelectric material is fixed to the surface of the structure that vibrates during service, so that vibrational energy in the structure is transferred into the piezoelectric material. Voltage leads from the piezoelectric material are connected to external electrical circuitry, where either a counter voltage is generated (an “active” vibration control technique) or the energy in the voltage produced by the piezoelectric material is dissipated (a “passive” vibration control technique). The active vibration control technique requires that the counter voltage be generated and fed back to the structure, and therefore requires more complex circuitry than the passive vibration control approach.
Passive piezoelectric shunting has been known for some time, but the existing techniques have drawbacks in their application. The efficiency of the external electrical circuitry can be improved and the available piezoelectric shunting methodology is also difficult to apply to reduce and to damp a number of different vibrational modes, which are usually present in complex structures. There is therefore a need for an improved approach to passive piezoelectric shunting of structures. Such an approach would be useful for aerospace structures, but would also find applications in a wide variety of other areas such as the tuning of acoustic components and structures.
Thus, a piezoelectric damping device solving the aforementioned problems is desired.